Silica has long been used as a reinforcing filler in rubber compounds for tyres, particularly treads.
Silica is used as a partial or total substitute for carbon black, for the advantages it affords in terms of rolling resistance and wet performance.
As is known, silica poses various drawbacks, due to the difficulty in dispersing it in the polymer base.
This is due to the presence on silica of surface silanol groups, which promote the formation of hydrogen bonds, and therefore silica particle clusters, and impart to the silica hydrophilic characteristics incompatible with the hydrophobic characteristics of rubber.
Obviously, poor silica dispersion in the compound results in a wide variation and lack of uniformity in the physical and mechanical characteristics, and especially poor wear resistance, of the compound. To solve the dispersion problem, silane bonding agents have long been used to prevent the formation of hydrogen bonds, by bonding with the silanol groups, while at the same time bonding the silica chemically to the polymer base.
Dispersing silica evenly in the polymer base, and therefore in the compound, calls for effective mixing. Normally, a first mixing stage, in which the polymer base is mixed with silica and the silane bonding agent, is followed by an additional mixing stage, in which nothing is added, and which serves solely to ensure correct silica dispersion. This additional mixing stage is followed by a final mixing stage, in which the curing agents are added and mixed with the mixture from the preceding stages.
Though improving silica dispersion, the additional mixing stage has the drawback of also stressing the polymer base, the repercussions of which will be clear to anyone skilled in the art. In other words, mixing alone is not enough to improve silica dispersion in the polymer base, without also compromising the characteristics of the polymer base itself.